call to probe_platform_engines! will break in Julia 1.8

[StefanKarpinski]

JuliaLang/Pkg.jl#2641 deletes the internal probe_platform_engines! which this package calls. This has been a no-op for since Julia 1.6: you can now just download stuff without calling that. As a result:

• Old versions of this package will break with Julia 1.8, so they will need to be capped in the registry
• To support both Julia ≤ 1.5 and ≥ 1.6 at the same time, check if the function exists before calling it
• To support only Julia ≥ 1.6 you can simply delete the call to probe_platform_engines!